Missing messages in Gmail folders on iPhone 4

Some of my Gmail IMAP folders are missing messages on my iPhone 4 (OS 5.0.1). In essence my phone doesn't reflect the content of the Gmail folder correctly.

This question has been asked a small number of times, so I know others have experienced it, but I haven't found a thread with an answer, so here goes again...

An example: I have a Travel folder (i.e., a Travel tag in Gmail speak) with airline ticket confirmations, itineraries, etc. When I open this folder on my iPhone, the messages are in chronological order but there are gaps of missing emails, sometimes weeks, months, or even years. My Travel folder might look like this on my phone:

1. Emails from today
2. Emails from yesterday
3. Emails from the day before yesterday
4. Emails from last month (seemingly selected at random)
5. Emails from 2004 (seemingly selected at random)
6. etc.

This makes it impossible to find a message from, say, last week, because it's missing in the gap between 3-4.

In folders with a small number of messages I can fetch them all by pressing "Load More Messages" a few times. Then all the gaps are filled in and everything on my phone matches what I see on the Web and in Mail.app. But some of my folders/tags have 3,000+ messages, so this is not a practical solution. (Of course everything is always in order in Mail.app because it fetches all messages and sorts them locally.)

Incidentally, I recently migrated from Yahoo! Mail to Gmail, so both accounts are exact clones of one another. I have both Yahoo! and Gmail set up (using IMAP) on my iPhone and the Yahoo! account does not have this problem. That is, when look at the Travel folder in my Yahoo! Mail account on my phone, it fetches the messages in the expected chronological order with no gaps, even in folders with thousands of messages.

Since I only see email gaps in Gmail folders and not Yahoo! on the same iPhone, I'm guessing it's more likely that this is an artifact of Google's IMAP than anything wrong or misconfigured with my phone, but I thought it couldn't hurt to ask here as well as in Google's forums.

Thanks.

P.S. I have deleted and re-created my Gmail account several times with the same problem.